Conventionally, as cold heat medium piping of a heat exchanger, water-supply/hot-water-supply piping, etc., there has been used a composite tube composed of a metal tube or a resin tube whose periphery is covered with a resin foam. In a known method of enhancing the heat insulating performance of such a composite tube, the expansion ratio of the resin foam is increased. It is believed that an increase in expansion ratio leads to enhancement in heat insulating property because of replacement of lots of resin by air, which has low heat conductivity.
As a method of enhancing the expansion ratio of a resin foam, a cross-linking foaming method is available. In the cross-linking foaming method, cross-linking of the resin is effected prior to foaming. Due to high tension of the resin, the method is characterized in that it involves little foam breakage and easily allows an increase in expansion ratio. However, to cover a tube with a cross-linking foam, it is necessary to cut the cross-linking foam in the form of a sheet into strips in conformity with an outer periphery of the tube, and to fuse together end portions of the sheet strips into to form a pipe-like configuration, which takes a lot of time and effort.
In an extrusion foaming method, the covering of the tube is completed simultaneously with extrusion of the resin foam out of a cross-head die, so less time and effort is required as compared with the cross-linking foaming method. However, the resin undergoes basically no cross-linking, so the tension of the resin is rather low, and the expansion ratio is increased not so easily as in the case of the cross-linking foaming method.
One of the factors leading to an increase in expansion ratio in the extrusion foaming method is the configuration of the die outlet. For example, as is known in the art, the expansion ratio is increased more easily when a bar-shaped foam is extruded out of an outlet with a circular section than when a tube-shaped foam is extruded out of an outlet with an annular (donut-shaped) section. This is considered due to the fact that if the sectional area is the same, the circular section has a smaller surface area than that of the annular section, which means the gas is diffused into the atmosphere less easily. In view of this, attempts have been made to obtain a tubular foam of high expansion ratio by bonding or fusing a plurality of bar-shaped foams to each other instead of extruding resin out of an annular die outlet to form a tubular foam. For example, a method is known in which resin is extruded in a bar-shaped (a thin-string-like) form out of a multi-hole die with circular outlets by the extrusion foaming method, and in which the resin bars are fused to each other through foaming to thereby cover a tube while forming a tubular foam (see, for example, Patent Document 1). [Patent Document 1] JP-A-60-85920 (“JP-A” means unexamined published Japanese patent application)